This invention relates generally to combustion of combustible fluids and, more particularly, to the combustion of waste fluid.
Numerous devices currently exist to combust fuels and waste materials. The devices include combustion systems, such as boilers and waste-to-energy facilities, which utilize the heat generated by combustion for the generation of process heat, steam or power. Other common combustion facilities include devices whose primary purpose is waste destruction, such as rotary kilns, multiple hearth incinerators, and fluidized bed incinerators. These devices are used to combust a wide range of materials, and they are typically able to handle low heating value waste materials, aqueous wastes, or physically hard-to-handle waste materials such as sludges. However, this capability comes at a high cost; these devices are mechanically complex, capital intensive, maintenance intensive, and they are usually fuel intensive as well when burning wastes containing little heating value. Liquid wastes, such as waste oils, are frequently used as fuels in industrial furnaces if they have a sufficiently high heating value. However, there are numerous liquid waste streams that cannot be used for this purpose because conventional burners will not produce a stable flame with them. These wastes become much more expensive to dispose of as a result, even though they contain considerable heat energy.
Aqueous wastes are by definition never used as fuels because they contain so much water. Disposal costs can be very high, especially if they must be incinerated. Currently they are simply sprayed into a furnace, where other fuels are combusted to supply the heat to evaporate the water. Sludges are particularly problematic because of their poor physical handling characteristics. They may have high or low heating values, but it is usually difficult to get them to burn because of their stickiness and tendency to clump up. For example, sludges from wastewater treatment systems are burned almost exclusively in multiple hearth furnaces or fluid bed incinerators, mainly because these furnaces can handle the sticky material without plugging up.
The current industry practice is to avoid the use of these poor quality waste materials as fuels. Typically these wastes can only be incinerated in specialized furnaces, such as rotary kilns, multiple hearth incinerators, and fluidized bed incinerators which are mechanically complex, capital intensive, maintenance intensive, and usually fuel intensive when burning wastes containing little heating value.
Rotary kiln incinerators tend to be mechanically complex and expensive to operate and maintain. Multiple hearth incinerators are designed specifically to handle sludges from wastewater treatment processes. They rely on mechanical arms to break-up the sludge, move it through the furnace, and expose it to flames. These incinerators are even more mechanically complex than rotary kilns, with associated high capital, operating and maintenance costs. Depending on the moisture content of the sludge, these incinerators may require large amounts of auxiliary fuel. Because of their specialized design, these furnaces are poor at handling variations in the waste materials, including variations in moisture content, volatile organic content, and physical consistency of the sludge. As an example, these furnaces have difficulty when fed grease-laden scum in amounts greater than a few percent of the total sludge feed. The scum, derived from wastewater skimming operations, causes smoking, high organic emissions, local overheating, and generally poor operability. At the other extreme, sludge that is much wetter than normal can lead to a drastic reduction in waste throughput, high fuel requirements, and difficulty in achieving complete destruction of the organics.
Fluidized bed incinerators make use of an inert bed of material that is fluidized with air from below. This design is suitable for incinerating wet materials because the turbulence and thermal inertia of the fluidized bed provides rapid drying of the moisture-laden waste. However, the design is mechanically complex and requires relatively large amounts of high pressure fluidizing air. Precise control must be maintained to achieve efficient incineration. The amount of fluidizing air must be carefully balanced against the mass of the bed, with too much air leading to attrition of particles and too little air causing loss of fluidization and local cold spots in the bed. The bed temperature must also be carefully balanced by control of the waste feed rate and auxiliary fuel feed rate. If the temperature gets too low, organic emissions become a problem, and if the temperature gets too high, fused ash may cause the bed to agglomerate and fluidization will be lost. Agglomeration of some types of sludge into large masses can also be a problem.
One way to handle hard to burn wastes and fuels is through the use of a pulse combustion system. When the chamber geometry and operating conditions of a combustion chamber are such that the acoustic, or pressure, waves generated during combustion are in phase with the energy release, a stable high frequency oscillating flow is formed. This oscillating flow can significantly increase heat transfer and reaction kinetics in reacting systems. When a pulsed combustor is coupled with an atomizing device, the pressure waves serve to atomize the fluid while the hot combustion products dry the droplets. Although capable of handling many types of materials, these systems must be very carefully designed and operated to maintain the correct phase relationship between the acoustic waves and the energy release.
Although easier to deal with than many waste fluids, other difficult to combust fluids such as heavy oil, coal-water slurries, orimulsion, and entrained solid fuels as well as conventional fuels can also benefit from the improved combustion system of this invention.
Accordingly, it is an object of this invention to provide an improved system for combusting waste fluid and other difficult to combust fluids.
The above and other objects, which will become apparent to those skilled in the art upon a reading of this disclosure, are attained by the present invention, one aspect of which is:
A method for combusting difficult to combust fluid comprising:
(A) contacting fuel with gaseous oxidant and combusting fuel with a portion of the gaseous oxidant to produce a hot combustion gas mixture containing gaseous oxidant;
(B) passing the hot combustion gas mixture through a nozzle to form a high speed combustion gas mixture having a steady flow;
(C) contacting the high speed steady flow combustion gas mixture with a flow of difficult to combust fluid and atomizing at least some of the flow of said fluid by the contact with the high speed steady flow combustion gas mixture; and
(D) combusting the atomized fluid by reaction with the gaseous oxidant of the high speed steady flow combustion gas mixture.
Another aspect of the invention is:
Apparatus for combusting difficult to combust fluid comprising:
(A) a hot combustion gas chamber, means for providing a non-pulsing flow of fuel into the hot combustion gas chamber, and means for providing a non-pulsing flow of gaseous oxidant into the hot combustion gas chamber;
(B) an atomization chamber and means for providing difficult to combust fluid into the atomization chamber;
(C) a nozzle positioned for receiving a steady flow of fluid from the hot combustion gas chamber and for ejecting a steady flow of fluid into the atomization chamber; and
(D) a combustion zone in flow communication with the atomization chamber.
As used herein the term xe2x80x9catomizingxe2x80x9d means to make in the form of many droplets or particles.
As used herein the term xe2x80x9cnozzlexe2x80x9d means a device having an input for receiving a fluid and an output for ejecting a fluid whereby the fluid exits the device at a higher velocity than it has when entering the device.
As used herein the term xe2x80x9cwaste fluidxe2x80x9d means a fluid typically containing organics, either as solids (sludges) or liquids, residue or byproduct that by its nature is not reused and must be disposed of.
As used herein the term xe2x80x9cdifficult to combust fluidxe2x80x9d means one or more of waste fluid, conventional fuels, heavy oil, coal-water slurry, orimulsion and entrained combustible solids.
As used herein the term xe2x80x9csteady flowxe2x80x9d means non-oscillating or non-pulsing flow, i.e. a flow of fluid wherein the bulk flow is continuously moving without rapid cessation or reversal of direction.